Over the past centuries, as we humans have cleared fields for farms, built roads and highways, and expanded cities ever outward, we’ve been cutting down trees. Since 1850, we’ve reduced global forest cover by one-third. We’ve also changed the way forests look: much of the world’s woodlands now exist in choppy fragments, with 20 percent of the remaining forest within 100 meters of an edge, like a road, backyard, cornfield, or parking lot.